The present invention relates to a worm support device and a power assist unit having the same.
Electric power steering apparatuses for automobiles are equipped with a power assist unit providing a steering assist power to a wheel steering mechanism. The power assist unit comprises a motor and a worm gear mechanism. The worm mechanism includes a worm shaft and a worm wheel. An appropriate backlash is provided in the engagement portion of the worm and worm wheel. Because of the backlash, when the worm is rotated in reverse, the tooth surface of the worm collides with the tooth surface of the worm wheel, producing a knocking sound. In order to overcome this drawback, an elastic body such as a spring or an O ring is inserted between the bearing for supporting the worm and the housing or worm (Japanese Patent Applications Laid-open No. H11-43062 and H11-171027).
In the above-described conventional example, an elastic body such as a spring or an O ring is required and the number of parts and assembling operations is increased. Further, changes in the elastic constant of the elastic body with time also cause a concern.